Aussie twins are world’s second semi-identical set

Rarity occurred when two sperm fertilised the same egg

A set of twins born in Queensland are more than just adorable. They’re a rarity. The brother and sister were born in 2014, but it took medical experts that long to figure out just exactly what happened to make their lives possible. They are only the second pair of semi-identical twins ever identified.

In the case of identical twins, one sperm fertilises an egg, which later splits into two embryos. Fraternal twins come about when two eggs are fertilised by different sperm. With semi-identical twins — first identified in a set in the US in 2007 — two sperm fertilise the same egg, which later splits into two embryos.

The first part of that is rare enough, since a sperm reaching an egg sends out a signal that typically prevents other sperm from getting to the target. When it does happen, the resulting embryo is usually inviable because it has three sets of chromosomes instead of two. In this case, three sets of cells formed. The first contained DNA from the mother and from the first sperm; the second contained DNA from the mother and second sperm; the third contained DNA from the two sperm, according to fetal medicine specialist Dr. Nicholas Fisk of the University of New South Wales and geneticist Michael Gabbett from Queensland University of Technology.

Gabbett and Fisk treated the mother and twins at the Royal Brisbane and Women’s Hospital. “The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” Fisk said in a statement. “However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

Whereas identical twins share 100 per cent of maternal and paternal DNA and fraternal twins have a 50-50 split of each, the “sesquizygotic” twins share 100 per cent of the mother’s DNA and 78 per cent on the father’s side, averaging out to 89 per cent identical, Gabbett told Gizmodo.

Sadly, the girl developed gonadal dysgenesis and had her ovaries removed as a precaution. She also had her right arm amputated as the result of a blood clot. Nevertheless, the twins are now aged 4 and otherwise happy and healthy.

“In terms of their physical development they’ve hit all their milestones,” Gabbett said.

“They’re two very cute, happy kids who happen to look much more similar than a regular brother and sister do. It just goes to show that biology is always surprising, and when you think you know it all, you don’t.”